This invention relates to a hydrogen iodide manufacturing method to be used for manufacturing hydrogen by way of a thermochemical decomposition process (IS method: iodine-sulfur method) and an apparatus to be used for such a method.
In recent years, the use of hydrogen is attracting attention as fuel that can suppress the emission of carbon dioxide (CO2) that is a greenhouse gas. The IS method is known as a technique of manufacturing hydrogen (See, for example, Japanese Patent Publication Nos. 60-52081, 60-48442 and 4-37002, and U.S. Pat. No. 4,127,644).
The principal reactions of the IS method include three reactions expressed by reaction formulas (1) through (3) shown below. Firstly, water, iodine and sulfur dioxide are made to react with each other at 70 to 100 degrees Celsius to produce hydrogen iodide to be used as raw material for forming hydrogen. Although sulfuric acid is also produced at this time, hydrogen iodide and sulfuric acid can be separated from each other by extracting the produced hydrogen iodide, using iodine by an amount of two to three times of the mass of the hydrogen iodide. In the second step, the obtained hydrogen iodide is thermally decomposed at 400 degrees Celsius to obtain hydrogen. In the third step, sulfuric acid is thermally decomposed at a high temperature of 900 degrees Celsius to recover the sulfur dioxide. Since iodine is obtained by thermally decomposing hydrogen iodide, it is reused with sulfur dioxide. The IS method is also referred to as thermochemical decomposition method, because water is decomposed into hydrogen and oxygen by means of sulfur dioxide, iodine and thermal energy by this method.I2+SO2+2H2O→2HI+H2SO4  (1)2HI→H2+I2  (2)2H2SO4→2SO2+2H2O+O2  (3)
The hydrogen iodide forming reaction of the reaction formula (1) is also referred to as Bunsen reaction. The efficiency of forming hydrogen iodide in the Bunsen reaction influences the downstream reactions and dominates the efficiency of hydrogen formation. The hydrogen iodide produced from the Bunsen reaction is extracted by excessively applying iodine, and is subsequently condensed by electrodialysis or distillation to form aqueous solution of hydrogen iodide that is concentrated beyond the azeotropic composition thereof. Hydrogen iodide gas can be obtained with ease by distillation from aqueous solution of hydrogen iodide whose concentration exceeds the azeotropic composition.
Hydrogen iodide gas is decomposed into hydrogen and iodine according to the reaction formula (2). The decomposing reaction of hydrogen iodide (Bodenstein reaction) is a uniform gas phase reaction. In the hydrogen iodide decomposing reaction, hydrogen iodide is dissociated to produce hydrogen and iodine at or near 400 degrees Celsius, so that hydrogen iodide, hydrogen and iodine coexist as mixed equilibrium gas at that temperature. The pressure equilibrium constant of the gas has been determined to be about 50 and the dissociation ratio is 22%. Therefore, it is important for the hydrogen manufacturing process to raise the hydrogen iodide forming ratio and obtain aqueous solution of hydrogen iodide beyond the azeotropic composition in the hydrogen iodide forming reaction.
While sulfurous acid and iodine quantitatively react with each other instantaneously, sulfur dioxide and iodine react with each other only by several percent stoichiometrically. The reason for this is that water is required for the reaction of sulfur dioxide and iodine, and that sulfur dioxide reacts with iodine only when it is dissolved in water to change itself into sulfurous acid. The equilibrium constant of the reaction of dissolving sulfur dioxide in water is 0.054. In view of this numerical value, the data is reasonable that the reaction of dissolving sulfur dioxide in water proceeds only by 8% at 55 degrees Celsius and only by 4% at 80 degrees Celsius relative to the reaction of sulfurous acid and iodine.
Additionally, while sulfur dioxide is a linear molecule and does not have any dipole moment, the sulfur atom in sulfurous acid is electrically positively charged. Therefore, the sulfur atom can easily interact with hydrogen iodide when sulfur dioxide is dissolved into water and turned to sulfurous acid. The reaction of sulfur dioxide, iodine and water proceeds faster at 55 degrees Celsius than at 80 degrees Celsius, because sulfur dioxide is dissolved less into water when the temperature is high, and because the reaction efficiency falls at 80 degrees Celsius due to iodine sublimation at that temperature.
When iodide ions exist in an aqueous solution system, iodine and iodide ions form a sort of complex to become more dissolved in water. As the iodine ion concentration rises, the iodine molecule grows to become I9− at largest.I2+I−→I331   (5)2I2+I−→I5−  (6)3I2+I−→I7−  (7)4I2+I−→I9−  (8)
Hydrogen iodide and sulfuric acid in the solution produced as a result of the Bunsen reaction are separated into two phases because of the difference of density as a result of addition of iodine. Iodide ions and iodine show affinity for each other, and it is known that associations as expressed by formulas (5) through (8) below take place when they coexist. The formation constants of the formulas are known. The formation constants for forming iodide ions/iodine complexes in aqueous solution of sodium iodide are listed below. Note, however, that β3 and β4 are estimated from the difference between β1 and β2 because no data is available about their values.log(β1/mol−1dm3)=log([I3−]/[I−][I2]/mol−1dm3)=2.86  (9-1)log(β2/mol−2dm6)=log([I5−]/[I−][I2]2/mol−2dm6)=5.27  (10-1)log(β3/mol−3dm9)=log([I7−]/[I−][I2]3/mol−3dm9)=7.23  (11-1)log(β4/mol−4dm12)=log([I9−]/[I−][I2]4/mol−4dm12)=8.75  (12-1)
The total iodide ion concentration CI− can be expressed by formula (13) below, and formulas (14) through (18) are obtained from CI− and the total equilibrium constants.CI−=[I−]+[I3−]+[I5−]+[I7−]+[I9−]  (13)
Thus, formula (9-2) below is obtained from the formula (9-1).[I3−]=[I−][I2]β1  (9-2)
Formula (10-2) below is obtained from the formula (10-1).[I5−]=[I−][I2]2β2  (10-2)
Formula (11-2) below is obtained from the formula (11-1).[I7−]=[I−][I2]3β3  (11-2)
Formula (12-2) below is obtained from the formula (12-1).[I9−]=[I−][I2]4β4  (12-2)
Thus,α0=[I−]/CI−=1/(1+[I2]β1+[I2]2β2+[I2]3β3+[I2]4β4)  (14)α1=[I3−]/CI−=[I2]β1/(1+[I2]β1+[I2]2β2+[I2]3β3+[I2]4β4)  (15)α2=[I5−]/CI−=[I2]2β2/(1+[I2]β1+[I2]2β2+[I2]3β3+[I2]4β4)  (16)α3=[I7−]/CI−=[I2]3β3/(1+[I2]β1+[I2]2β2+[I2]3β3+[I2]4β4)  (17)α4=[I9−]/CI−[I2]4β4/(1+[I2]β1+[I2]2β2+[I2]3β3+[I2]4β4)  (18)
Thus, α0 through αa4 show the distribution of formation of the respective iodide ions/iodine complexes. From the distribution of formation curves, it is understood that iodide ions and iodine can readily interact and form complexes. Where free [I2] shows a large proportion, iodide ions are bonded to iodine to form complexes of higher orders. Therefore, hydrogen iodide is stably bonded to iodine in the lower phase of HI/I2 that is produced as a result of a two-phase separation process. The yield of iodide ions rises under the condition of little water, probably because the iodine concentration relatively rises to form complexes of higher orders, which then move to the lower phase.
To obtain gaseous hydrogen iodide by means of the prior art, hydrogen iodide and sulfuric acid are subjected to a two-phase separation process that uses iodine after the completion of a Bunsen reaction, and subsequently the lower phase liquid is moved to a hydrogen iodide refining process so as to remove the sulfuric acid contained in the lower phase liquid by causing a Bunsen reaction to take place in the opposite direction. In this stage of operation, the weight percent concentration of hydrogen iodide (=(mass of hydrogen iodide contained in lower phase liquid)/(sum of mass of hydrogen iodide and water contained in lower phase liquid)) cannot exceed the azeotropic composition of hydrogen iodide. Then, it is not possible to directly take out hydrogen iodide anhydride gas with ease. Therefore, the liquid product that contains hydrogen iodide is condensed by electrodialysis until it comes to exceed the azeotropic composition. The azeotropic composition of hydrogen iodide is 57%.
Hydrogen iodide can be isolated with ease from aqueous solution of hydrogen iodide that is condensed beyond the azeotropic composition by means of electrodialysis. Therefore, it is possible to take out pure hydrogen iodide gas by distillation in the next step. If the hydrogen iodide concentration in the lower phase liquid can be made to exceed the azeotropic composition as a result of a two-phase separation process that is conducted after the completion of a Bunsen reaction, the concentration step of electrodialysis is not necessary.
Sulfur dioxide dissolves by only 0.8 moles in 1 liter of water under the conditions of room temperature and atmospheric pressure, and free sulfur dioxide reacts with water and become decomposed to produce sulfur. One of the factors that lower the hydrogen iodide forming efficiency is clogging of pipes due to deposited sulfur. The efficiency of hydrogen iodide formation rises when sulfur dioxide is forcibly mixed with water that operates as solvent to increase the contact time with iodine and the rise of the efficiency of hydrogen iodide formation is reflected to the efficiency of forming hydrogen. However, once the solubility of sulfur dioxide in water is exceeded, sulfur dioxide is no longer dissolved in water and free sulfur dioxide reacts with water and becomes decomposed to produce sulfur, which by turn clogs the pipes of the mixer. Additionally, since the reaction of forming hydrogen iodide is conducted at or near 100 degrees Celsius, loss of iodide due to sublimation and clogging of pipes are problems that need to be solved in the industry.
Since hydrogen iodide can be obtained with ease by causing iodine to react with aqueous solution of sulfurous acid and an oxidation/reduction reaction to take place, the object of the present invention is to raise the conversion ratio of sulfur dioxide into hydrogen iodide or sulfuric acid, and thereby to obtain highly concentrated aqueous solution of hydrogen iodide by improving the solubility of sulfur dioxide relative to water in a hydrogen iodide forming reaction.